This invention relates to start-up power supplies for television receivers.
Rectified AC line voltage is filtered and regulated to provide a high B+ operating voltage to the television receiver. The high B+ may be coupled, for example, to the horizontal deflection circuit for generating scanning current in the horizontal deflection winding. Low B+ voltages must also be provided as operating voltages to various receiver circuits, such as the oscillator and driver stages of the horizontal deflection circuit itself.
In U.S. Pat. No. 3,980,821, granted to J. C. Peer et al., there is disclosed a high voltage protection circuit which provides a continuous low B+ operating voltage to the horizontal oscillator of a horizontal deflection circuit. The low B+ is derived from the steady-state voltage that is available across a secondary winding coupled to a rectified AC line choke input filter. If the choke shorts, the low B+ voltage decreases to an unuseable level.
Often it is desirable to derive the low B+ voltages from a secondary winding of the horizontal output transformer after the horizontal deflection circuit has begun to function. However, during the initial start-up interval after the receiver is turned on, no low B+ voltages are generated by the horizontal output transformer to operate the oscillator and driver stages of the horizontal deflection circuit. Another source of low B+ voltage must be provided during this initial start-up interval.
One method of providing a low B+ start-up voltage, as disclosed in U.S. Pat. No. 3,621,134, granted to R. J. Waring, couples a regulated switching transistor in series with the high B+. The low B+ is obtained at the emitter of the switching transistor. The base-emitter of the transistor becomes reverse biased after the initial start-up period. However, in such an arrangement, the low B+ is not isolated from the main power supply and would be unsuitable for driving circuits which are isolated from the main power supply.
Another method, as disclosed in U.S. Pat. No. 3,947,632, granted to R. J. Giger et al., couples a secondary winding to the degaussing coil of a television receiver. A thermistor, in series with the degaussing coil, is of low resistance during initial turn-on of the receiver, and a relatively large AC potential is developed in the secondary winding for generating a start-up voltage. While an isolated voltage can be derived from the secondary winding, the winding structure is relatively uneconomical.